Strip transmission lines



Nov. 17, 1959 E. G. FUBVINI ETAL 2,913,686

STRIP TRANSMISSION LINES Filed Sept. 17. 1953 2 Sheets-Sheet 1 Fig. 5

- INVENTOR.

6065M. G. Fl/anw wmwao 45. FR OMM t I M A TORNEY Nov. 17, 1959 E. G. FUBINI YET AL 2,913,686

STRIP TRANSMISSION LINES Filed Sept. 17, 1953 2 Sheets-Sheet 2 I 2 NEUTRAL PLANE Fig. 5

INVENTOR. EUGENE aids/#1 W/NF/ELD E- FROM/II ATTORNEY United States Patent "Cutler-Hammer, fire, Milwaukee, Wis., a corporation *of Delaware ApplicationSept embcr 17, 1953, Serial No. 380,674

11 Claiins.- (Cress-s4) This iriventio'n'relates to transmissionlines, and in particular to strip transmission lines.

Inmariy applications of strip transmission lines in radio frequency systems, particularly in the microwave region of thefrequency spectrum, dielectric losses prove to be a limitation from the standpoint of economic andetlicient operation. Moreover, in some utilizations, it is demanded that the effective electrical lengthof the circuit components remain substantially uninfluenced by changes in-the ambient temperature, so that performance will be of a constant and dependable nature. Also, it is important that undesired radiation losses be kept to a minimum. In the microwave field in particular, it is im. portant that these criteria be realized without any sacrific'e of the requirements for light weight and miniaturization.

The/"advantages of strip transmission line microwave ccr'npbnents-are'by nowwell known. Among these are: lightweight, easeof manufacture, cheapness, miniaturization, etc. The possibility of usingprinted circuit techniques-for the--coristruction of 'microwave circuits invowing-stri transmission lines hasbe'en disclosed by Barrett, Electronics, June 1952, and D. D. Grieg ami H. F. Engel'mann, Proc. 1. R. E., December 1952.

In general, these publications depict three types of ti'a'ri'smis'sion lines. The; closed strip or dielectric sandwieh transmission line comprises a narrow ribbon conductor immersed in dielectric material, thewholebeing bounded by fiat metallic plates. The second" type, the open-striptrarismission line,-comp'rises a ribbon conductor separatednom a metallic ground 'pla'neby a"'di'electri'c 'sla'b. The third type, called the wirerabove-grou'n'd transmission line, consists of a wire or cylindrical. conductor suppofted above a ground planeby dielectric sup' ports suitably positioned at spaced intervals' longitudinally of th'e line.

n It is an objectbf this invention to provide a strip transmission linehaving low dielectric and undesired radiation losses.

It-is anotherbbject ofthe invention to providea-strip transmission line which has an effctive electrical length which is substantially uninfluenced by changes inla'mbient temperatures. t

It is'anotherobject to provide a strip transmission line which is lightweight, convenient to fabricate, and which can be arranged to occupya relatively small amountof space. I p

In accordance "with the present invention, a transmis-' sion line is"pr'ovided'comprising apair of metallic ground plates disposed in parallel relation to each other, and at an e'ifective electrical-distance of less than one halfiwave length of the highest frequency of the energy to be propagated. A thin sheet of dielectric material is supported in" spaced relation between the plates, the dielectric having conducting nieans suitably supported thereon. The conducting means cooperates with the bounding plates in guiding passage of the radio frequencyenergy. The'above as well-as fur ther-objects of the invention "ice 2 will be more readily understoodfrom the following description taken in conjunction with the accompanying drawings wherein:

Figure 1 is a perspective view of a section of strip transmission line in accordance with one illustrative embodiment of this invention;

Figure 2 is a flux plot showing the electric field lines associated with conducting means positioned with perfect symmetry between bounding plates;

Figure 3 is a flux plotshowing the distortion of the electric field lines as a result of offsetting the conducting means, t

Figure 4 is a'perspective view of a section of strip transmission line in accordance with another illustrative embodiment of this invention, and

Figure 5 is a view, partially in longitudinal cross-section, of a section of striptransmission line of the invention, with coaxial line fittings connected thereto.

Referring now to Figure 1, there is disclosed one illustrative embodiment of astrip transmission line in accordance with my invention, for use in a radio frequency system. Reference numeral 1 denotes a thin sheet of dielectric material positioned in spaced relation between a pair of metallic ground plates 2, by means of metallic or dielectric support members 3. The spacing 4 between the groundplates 2'should not exceed an electrical length of A min/2, that is, one half wavelength of the highest frequency of the energy to be propagated, and much smaller spacings have been found advantageous and are commonly used in practice. A conducting means, shown generally at 5, for guiding passage of the radio frequency energy along the transmission line is rigidly secured to the thin dielectric sheet 1. The conducting means comprises a strip of metallic conductor material 6 which may be conveniently fashioned by printed circuit techniques. As willbe noted, in'Figure 1' the conducting'means 6 is'positione'd on only one side of the dielectric.

The support members 3 are arranged atconvenient location's, far enough removed from the lateraledges of the conductor '6 so as to be substantially outside the regionwhe'rethe electric field lines are concentrated; Roughly speaking'the distance 7 between the members Sand'thenear'edges of conductor 6 shouldbe'equal'to, or greater than, the distance 4 between the ground plates 2. I

At this point some of the advantages of the transmission-"lin'e just described over prior art types will be obviousjwhile other advantages will necessitate further "discussion.

As is known, the velocity of propagation and wavelength of'hi ghfrequencyenergy; passing through a dielectrio is dependent upon the nature ofthe dielectric material, 'and 'morespecifically upon the dielectric constant ofthedielectric' medium. Further, the dielectric constant itself is a function of the ambient temperature, so that with variations in' temperature, the effective electrical length of the circuit elements will be changed. These variations in electr-ical-length are particularly undesirable and troublesomein the dielectricsandwich and open'strip transmission line types Moreover, the attenuation in astrip transmission li'ne is increased by the presence of the dielectric material. In the line. shown in Figure 1,the' dielectric material 1 is kept at a minimum consis'tentwith the requirements for structural rigidity, so that the eif'ectiveelectrical lengths ofthe-circuitcompo'nents remain substantiallyconstantthroughout the ."range of temperature changes experienced under practical operating conditions. In addition, dielectric losses are largely eliminated.

In-Figure Z there is-shown a flux plot (TEM mode) in a lateral cross-sectionaL-plane of-the transmission: line of Fig. 1.. In. order to simplify thediscussion, only-the electric field lines have been shown, it being undera 3 stood, that the magnetic field lines are everywhere normal to the electric field lines. For the same reason, the thin dielectric has been removed from the figure. As will be apparent from a study of Figure 2, perfect field symmetry prevails, and the electric field lines do not cross a plane exactly centered between the ground plates and parallel thereto (designated Neutral Plane). The outer regions 8, 9 tend to become field free because the mode existing in the central region is beyond cutoff 'in regions 8 and 9. Stated differently, with the ground plate spacing 4 less than a half-wavelength, as heretofore mentioned, in the regions 8 and 9 lateral of the conducting means the ground plates function as a waveguide beyond cutoff for the fundamental TEM mode of propagation illustrated in Figure 2, so that the fields in the lateral directions are rapidly attenuated. For this reason it is possible to effect great savings by arranging several independent circuits in close physical proximity without obtaining troublesome interference between adjacent lines (provided no mode is excited which is capable of propagating between the bounding ground plates). It is usually desirable to operate in the fundamental TEM mode of propagation. In order to discourage higher frequency modes it is found advantageous to select the width W of the conductor 6 so that the sum of the width of the strip plus the spacing 4 between the ground plates is less than a half-wavelength of the energy to be propagated, although theory indicates a slightly more lenient condition.

In practice there may be some departure from the exact centered parallel position of the center conductor between the ground planes, illustrated in Figure 2. In such case the electric field pattern will be distorted from that shown in Figure 2, and undesired modes of propagation may be excited. The nature and amount of such departures which are permissible depend upon the performance desired of the line, and upon the particular application in which it is used.

One such type of departure is offsetting, wherein the central conductor is closer to one ground plate than to the other but remains parallel thereto. This is illustrated in Figure 3.

Referring to Fig. 3, as the conducting means 5 is moved closer to one bounding plate than to the other, that is, the conducting means is offset, then the tendency is for more and more electric field lines to terminate on the nearer plate. In order to illustrate this effect, an exaggerated offsetting of the conducting means 5 is shown in Figure 4. Very few of the electric field lines terminate on the lower plate, so that the field pattern is distorted and an undesired mode can be excited. It has been found from experiment that as much as 10% offset in one direction or the other can be tolerated in most applications.

In some applications the excitation of undesired modes can be tolerated, and the amount of coupling to an undesired mode will depend upon the problem encountered. For example, When very high Q networks are necessitated, the coupling between different parts of the circuit plumbing and the undesired mode may be unacceptable, even if very small by ordinary standards. Roughly speaking, one can state that the amount of coupling that may be tolerated when the Q is 1000, is one where the undesired mode is A (60 db) of the desired mode. As a trough rule of thumb it has been found that acceptable coupling is inversely proportional to the maximum Q desired.

The closed stn'p transmission line has a characteristic impedance (Z which, to a first approximation, is independent of the exact centering of the inner strip conductor between the ground plates. This is not the situation in the open strip and the wire-above-ground transmission lines where the characteristic impedance (Z is a function of the distance between the conductor and the ground plate. However, in the structure of Figure 1, it

may at times be desirable to off-center the conducting means slightly in order to compensate for the presence of the conducting means 5 on only one side of the dielectric.

The open strip transmission line and the wire-aboveground line of the prior art are of course limiting cases of complete off-centering, since one of the ground plates is at infinity with respect to the other. The desired and the undesired modes then coincide, and the electrical coupling between adjacent circuit elements is such as to prevent their utilization in many applications, such as for example, high Q circuits and feeds for antenna systems. This is of course a serious limitation, particularly in application to strip transmission line plumbing system. The transmission line of Figure 1 on the other band possesses no such design limitations.

Another illustrative embodiment of the invention is disclosed in Figure 4. The structure shown is similar'to that shown in Figure 1, except that the conducting means 5 is formed by two conductive strips 6, 6 fixedly secured to opposite sides of the thin dielectric sheet in registered relationship. Thus the strips 6, 6' normally operate electrically as a single conductor. This embodiment has the additional advantage of being inherently symmetrical, since the conductors 6, 6' are equidistant respectively from the ground plates 2, with only air-dielectric between respective conductors and ground plates. Another advantage derives from the fact that the thin dielectric 1 lies substantially in a neutral plane in so far as the electric field between the bounding plates 2 is concerned, and in a substantially zero electric field between conductive strips 6, 6, so that dielectric losses are reduced to such an extent as to be almost entirely eliminated.

The above discussion concerning departures from an exactly centered parallel position of the central conducting means 5 applies also to the embodiment of Figure 4.

In the usual operation of the line shown in Figure 4, the conductors 6, 6' are fed in parallel. In such case spurious resonances may be introduced. In order to avoid this result, the opposed conductors 6, 6 are shown electrically connected by means of metallic connectors 11 spaced longitudinally along the transmission line. Advantageously these connectors 11 may be rivets, metalized holes or the like for establishing the requisite electrical contact between the conductors. The purpose in establishing electrical contact between opposed conductors 6, 6 is to maintain them at the same potential at each cross-section, so that spurious resonances cannot occur. To assure freedom from such spurious resonances, the distance 12 between adajcent connectors 11 may be made less than one-half wavelength at the highest operating frequency.

With the transmission line shown in Figure 4, circuit elements have been designed with Qs equal to or higher ihan those obtained with conventional air-filled coaxial mes.

Figure 5 shows a conventional manner of coupling a strip transmission line to ordinary coaxial line. Here each end of a strip transmission line section 13 is provided with coaxial line fittings 14, 14' of conventional construction. As shown, the strip transmission line section 13 is constructed as in Figure 4, with the central conducting means comprising a pair of flat conductive strips 6, 6' afiixed to opposite sides of the dielectric sheet 1 in registered relationship. The dielectric sheet is supported by members 3 in parallel relationship with the ground plates 2 and midway therebetween.

The coaxial fitting 14 has its outer conductor 15 connected to the ground plates 2 in any suitable manner, here shown as by flanges 16, 16 which are bolted together. The conductor 17 of the coaxial fitting is centrally supported in outer conductor 15 by any suitable means, here shown as an insulating ring 18. This central conductor 17 is conductively attached to the two conductive strips 6, 6'. The coaxial fitting 14 is similarly as above described. Asis common :practice, the coaxial The parallel feeding of strips 6, 6 causes corresponding ends thereofto be at the same relative potential at all times, and thus, in the absence of spurious resonaiices, causes opposed areas of the strips throughout the length thereof to be at substantially the same potential. However, if spurious resonances are troublesome, the strips may be electrically connected at spaced points along the length thereof by rivets, 'etc., as described in fittings are threaded to permitready attachment of coconnection with Figure 4. v

As used in this specification, and in the claims to {follow, the term transmission lineis used in the broadest sense accordance with current us'agein the radio frequency' ar-t. r Thus in contemplation 'of the instant invention, the transmission-line may beus'e'd to transmit energy as a 'res'bnant circuit, as-a-circuitelement such as a wave filter, 'inetallic-insulators-or-the like, as an aid in impedance matchingetct- T i It should be understood that the embodiments of the invention are illustrative only, and other modifications within the-scope ofthe' appended claims will occur to those skilled in the anrrdm a consideration of the structures shown togetherwith the teachings hereof.

1. A microwave transmission line which comprises a pair of spacediisubstantially parallelconductive surfaces forming ground platesjthe-spacing; of-saidg round plates being less than a half-wavelength of the highest frequency of the energy to be propagated, a central conducting means including at least one elongated conductive surface afiixed to a sheet of dielectric and positioned substantially midway between said ground plates, said dielectric sheet being spaced from both ground plates and having a thickness small compared to said spacing of the ground plates, the space between the portions of the ground plates opposed to said central'conducting means and extending laterally a substantial distance on both sides thereof being substantially air-dielectric except for said dielectric sheet and central conducting means, said central conducting means cooperating with said ground plates to transmit radio frequency energy along the transmission line, both of said ground plates extending laterally beyond the lateral edges of said conducting means a distance at least as great as the spacing of said ground plates.

2. A microwave transmission line according to claim 1 wherein the central conducting means comprises a pair of metallic conductors mounted on opposite sides of the dielectric sheet and operating electrically as substantially a single conductor.

3. A microwave transmission line according to claim 1 in which the conducting means comprises a flat conductor having a plane surface substantially parallel to said pair of plates.

4. A microwave transmission line according to claim 1 in which the central conducting means comprises a pair of flat conductors mounted on opposite sides of said dielectric and having respective plane surfaces substantially parallel to said ground plates, said pair of flat conductors operating electrically as substantially a single conductor.

5. A microwave transmission line according to claim 1 in which the thin dielectric material is supported in a region lateral of said central conducting means and substantially free of electric field lines.

6. A microwave transmission line according to claim 1 in which the central conducting means comprises a pair of flat conductors mounted on opposite sides of said dielectric and having respective plane surfaces substantially parallel to said pair of ground plates, and a plurality of metallic connectors, said connectors passing through the dielectric material and electrically interconnecting the said conduetorsat spaeed iritenvals, said intervalsheing lessrthan one halfwave length-at said highest-frequency. A microwave transmission line which comprises a pair of spaced substantially parallel conductive surfaces forming ground plates, the spacing of said ground plates being less than a half-wavelength of the highest -fre'- quency of the energy to. be. propagated, a central conducting means including at least one elongated flat conductive surface afiixedto a sheet of dielectric and positioned substantially midway between said ground plates and substantially parallel therewith, said dielectric sheet being spaced fromboth ground plates and having a thickness small compared to-said spacing of theground-plates, the space betweentheportions of theground pl-ates ;op posed to'said central conducting means and extending laterally a substantial distance on both sides thereof being substantially;air-dielectric except for said dielectric sheet and central conducting means, said central conducting means cooperating with said ground plates to transmit radig frequency energy along the transmission line inthe TEMiirnode of. propagation, said-1 ground plates extending laterally beyond-the lateral edges ofsaid cent'ralconducting-means; adistance'at least-as great as the spacingfofsaid ground plates. 7

A microwave transmission line which comprisesa pair of spaced substantially parallel conductive surfaces formingtground pIates -the spacingof said ground plates being less than; a half-wavelength ;of the, highest frequencyof the energy to be..propagated,- a sheet of dielectric-positioned substantially midway between said kground plates and substantially'fparallel therewith and {spaced therefrom, a pair of elongated fiat conductive surfaces aflixed to opposite sides of said dielectric sheet in registered relationship with opposed areas at substantially the same potential to form a central conducting means, said central conducting means cooperating with said ground plates to transmit radio frequency energy along the transmission line, said ground plates extending'laterally beyond the lateral edges of said central conducting means a distance at least as great as the spacing of said ground plates.

9. A microwave transmission line which comprises a pair of spaced substantially parallel conductive surfaces forming ground plates, the spacing of said ground plates being less than a half-wavelength of the highest frequency of the energy to be propagated, a sheet of dielectric positioned substantially midway between said ground plates and substantially parallel therewith, the thickness of said dielectric sheet being small compared to the spacing of said ground plates, a pair of elongated fiat conductive surfaces affixed to opposite sides of said dielectric sheet in registered relationship with opposed areas at substantially the same potential to form a central conducting means, said central conducting means cooperating With said ground plates to transmit radio frequency energy along the transmission line in the TEM mode of propagation, said ground plates extending laterallybeyond the lateral edges of said central conducting means a distance at least as great as the spacing of said ground plates.

10. A microwave transmission line which comprises a pair of spaced substantially parallel conductive surfaces forming ground plates, the spacing of said ground plates being less than a half-wavelength of the highest frequency of the energy to be propagated, a central conducting means including at least one elongated flat conductive surface applied to a sheet of dielectric and positioned substantially midway between said ground plates and substantially parallel therewith, said dielectric sheet being spaced from both ground plates and having a thickness small compared to said spacing of the ground plates, the space between the portions of the ground plates opposed to said central conducting means and extending laterally a substantial distance on both sides thereof being substantially air-dielectric except for said dielectric sheet and central conducting means, said central conducting means cooperating with said ground plates to transmit radio frequency energy along the transmission line with the portions of said dielectric sheet lateral of said central conducting means lying substantially in the neutral plane of the electric field for the TEM mode of propagation, both of said ground plates extend ing laterally beyond the lateral edges of said conducting means a distance at least as great as the spacing of said ground plates. 7

11. A microwave transmission line which comprises a pair of spaced substantially parallel conductive surfaces forming ground plates, the spacing of said ground plates being less than a half-wavelength of thehighest frequency of the energy to be propagated, a sheet of dielectric positioned substantially midway between said ground plates and substantially parallel therewith, said dielectric sheet being spaced from both ground plates and having a thickness small compared to said spacing of the ground plates, a pair of elongated flat conductive surfaces afiixed to opposite sides of said dielectric sheet in registered relationship with opposed areas substantially the same potential to form acentral conducting means, the space between the portions of the ground plates opposed to said central conducting means and extending laterally a substantial distance on both sides thereof being substantially air-dielectric except for said dielectric sheet and central conducting means, said central conducting means cooperating with said ground plates to transmit radio frequency energy along the transmission line with the portions of said dielectric sheet lateral of said central conducting means lying substantially in the neutral plane of the electric field for the TEM mode of propagation, both of'said ground plates extending laterally beyond the lateral edges of said conducting means a distance at least as great as the spacing of said ground plates, and supporting means for said dielectric sheet located laterally of said central conducting means a distance at least as great as the spacing of said ground plates.

References Cited in the file of this patent UNITED STATES PATENTS 2,171,219 Malter Aug. 29, 1939 2,231,602 Southworth Feb. 11, 1941 2,287,502 Togesen June 23, 1942 2,441,960 Eisler May 25, 1948 2,676,309 Armstrong -2 Apr. 20, 1954 2,794,185 Sichak May 28, 1957 12,800,634 Greig July 23, 1957 2,810,892 Blitz Oct. 22, 1957 2,812,501 Sommers Nov. 5, 1957 FOREIGN PATENTS 601,514 Great Britain May 7, 1948 OTHER REFERENCES Publication I, Barrett, Etched Sheets Serve as Microwave Components, Electronics, June 1952, pp. 114-118.

Publication II, Greig, A New Transmission T echnique for the Kilomegacycle Range, Proc. of the I.R.E., December 1952, pp. 1644-1650.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No'. 2,913,686 November 17, 1959 Eugene Ge Fuhini et n10 It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as. corrected below;

Column 1., line 17, beginning with "In many applications" strike out the "entire paragraph ending with "and miniaturizationg" in lines 29 and 30, same column, and insert the same after the paragraph ending at line 50, same column; line 69, before "having" insert material 80111111113, line 1, after .st'ood" strike; out the some; line 64, for "trough" readrough o'OlUJ I A, line-1.4,ior system read. syetems.---.-; line 15., for "band read hand i Signed and sealed 3rd day of Niey 196%,

(SEA Attest:

KARL minim 9 ROBERT c. WATSON Attesting Officer Commissioner of Patents 

